I just think the bars look really clean with just a pair of brake levers ...
I take my SS out when I'm riding with slower people. It started because one of them said I was only faster because I had full suspension and disk brakes. So next time I turned up on a rigid SS.
No, it's not as quick on the flats or the downhills.
Yes, it's light (but then it would be anyway, it's a Merlin Mountain with RC31s) saving the mechs, extra chain, cables, shifters (but not the chainrings as I haven't bought any SS bolts yet).
Why ride it? The same reason that I'd drive a TR6 if I had that and a Porsche 911 in the garage: because sometimes it's not about getting there fast, it's about the journey.